This invention relates in general to the field of integrated circuits, and more particularly to bipolar thin-film transistors and method for forming same.
Thin-film transistors are used in a variety of electronic devices such as high performance active matrix liquid crystal displays, two dimensional medical imagers, sensors, artificial retinas, ring oscillators, and high energy particle detectors. Conventional thin-film transistors are field-effect transistors. Field effect transistors contain three electrodes, source, a drain, and a gate. During transistor operation, current flows from the source to the drain, and the magnitude of that current is controlled by a voltage applied to the gate. The thin-film transistor performance is significantly influenced by characteristics of the interface between the semiconductor and an adjacent dielectric.
These thin-film transistors are generally made from amorphous silicon or polycrystalline silicon. An amorphous silicon thin-film transistor can be fabricated in an array form on large substrates (such as 600 mm by 720 mm glass) at low temperature (such as less than 350xc2x0 C.) with high throughputs. A polycrystalline silicon thin-film transistor generally utilizes a high temperature crystallization, or deposition step, which restricts the number of suitable substrates. Although there are low temperature polycrystalline silicon thin film processes, such as excimer laser crystallization and solid phase crystallization methods, processing time for such methods is usually excessive as compared to the requirements for mass production on very large substrates.
There are two major drawbacks with conventional amorphous silicon thin-film transistors: high photoleakage and low field-effect mobility. The high photoleakage problem has been addressed by the addition of a light blocking layer, such as a metal or a black polymer film, above the thin-film transistors. Conventional thin-film transistors have a maximum mobility of less than about 1.5 cm2/Vs. This level of mobility is proper for some simple local operations such as the pixel switching of a LCD; however, the low mobility greatly restricts the application of thin-film transistors to many advanced products. For example, amorphous silicon thin-film transistors generally cannot be used to build driving circuits of a LCD. Further, many electronic designs require a higher mobility, such as greater than 10 cm2/Vs.
A bipolar transistor may have a higher operating speed than a conventional field-effect transistor (FET), such as a metal-oxide-semiconductor (MOS) transistor. Conventional bipolar transistors are fabricated based on a single crystal silicon wafer. One of the major yield loss factors is the shortening of the emitter and collector regions (also known as piping) due to base-region crystal defect caused by contamination or other factors. Conventional bipolar transistors are prepared on an epitaxy film that is deposited at a high temperature, e.g., above 950xc2x0 C. The out diffusion of the dopant from the collector region greatly limits the minimum base region thickness. Therefore, due to the high temperature, the base and collector concentration profiles are not abrupt.
In accordance with the present invention, a bipolar thin-film transistor and method for forming same is provided that addresses disadvantages and problems associated with conventional transistors.
According to one embodiment of the invention, a bipolar transistor includes a substrate, a first layer, a second layer, and a third layer. The first layer comprises non-single-crystalline semiconductor material having a first conductivity type deposited on the substrate. The second layer comprises non-single-crystalline semiconductor material having a second conductivity type deposited on at least a portion of the first layer. The third layer comprises non-single-crystalline semiconductor material having a conductivity type different than the second conductivity type deposited on at least a portion of the second layer. The first, second, and third layers form a collector, base, and emitter of the bipolar junction transistor.
According to another embodiment of the invention, a method for fabricating a bipolar transistor includes depositing a first layer of semiconductor materials to form a first transistor region, depositing a second layer of semiconductor material over at least a portion of the first layer to form a base of the transistor, and depositing a third layer of semiconductor material over at least a portion of the second layer to form a third transistor region.
The present invention provides various technical advantages over current thin film transistors including higher mobility, higher yield and greater flexibility in transistor structure. In addition, the present invention also has advantages over conventional silicon wafer based thin film transistors including higher yield, lower process temperatures, and less restrictions on size and material of substrates.
Other advantages may be readily ascertainable by those skilled in the art and the following figures, description, and claims.